You could try storing some string value in the Cache object on a button click, and then use another button that will retrieve the previously stored object in the Cache. If the value is null, when testing under Firefox, then I suppose the problem is something general that is not related to RadCaptcha. The following code line will add the string "CacheValue" to the Cache.
, DateTime.Now.AddSeconds(Convert.ToDouble(5 * 60)),TimeSpan.Zero, CacheItemPriority.NotRemovable,
The same test can be performed with the Session object.
In case the problem persists, please send detailed information about your server configuration, and sample project to reproduce the issue locally. Moreover, if possible please provide a live URL, where we can test the Captcha and see if we are getting the same behavior.
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